The principle governing the operation of the read sensor in a magnetic disk storage device is the change of resistivity of certain materials in the presence of a magnetic field (MR or magneto-resistance). Magneto-resistance can be significantly increased by means of a structure known as a spin valve. The resulting increase (known as Giant magneto-resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of the solid as a whole
The key elements of what is termed a top spin valve are, starting at the lowest level, a seed layer, a free magnetic layer, a non-magnetic spacer layer, a magnetically pinned layer, a pinning layer, and capping layer. When this order of layering is Inverted the resulting structure has the free layer at the top and is termed a bottom spin valve. The present invention is concerned with the latter type. One advantage of this type of design is that the capping layer, if made of certain materials, can, in addition to protecting the GMR stack from corrosion, also bring about more specular reflection at the free layer-capping layer interface, thereby increasing the conductance. Isolation of the device from extraneous magnetic fields is achieved by sandwiching it between two magnetic shield layers.
Although the layers enumerated above are all that is needed to produce the GMR effect, additional problems remain. In particular, there are certain noise effects associated with such a structure. As first shown by Barkhausen in 1919, magnetization in a layer can be irregular because of reversible breaking of magnetic domain walls, leading to the phenomenon of Barkhausen noise. The solution to this problem has been to provide a device structure conducive to single-domain films for the GMR sensor and to ensure that the domain configuration remains unperturbed after processing and fabrication steps and under normal operation. This is most commonly accomplished by giving the structure a permanent longitudinal bias provided by two opposing permanent magnets.
Today, the most common sensor stabilization scheme uses a hard bias abutted junction structure as illustrated in FIG. 1. Seen there are substrate 11, seed layer 12, and layer 13 which represents an antiferromagnetic pinning layer as well as a pair of antiparallel pinned layers. Layer 16 is a copper spacer layer, layer 17 is the free layer, and layer 18 is a capping layer. The read element width is defined by the edges that were milled out by etching. These edges are stabilized through the magneto-static coupling provided by the adjacent hard bias layer 14 which is separated from the sensor by non-magnetic seed layer 12. This hard bias layer is traditionally similar to those magnetic media materials which offer large coercivity, typically several thousands Oesterds.
The major problem of this scheme is that it breaks the magnetic continuity of the GMR sensor, and so cannot avoid magnetic charge accumulation at the edge of the sensor area, making a coherent spin rotation during sensing difficult to achieve without proper biasing. Traditional methods require a large magnetic moment to be put on the edge and utilize magnetostatic coupling to stabilize the edge spins. As the sensor size shrinks, these extra moments stiffen the whole sensor thus reducing sensor sensitivity to the media field. On the other hand, without these extra moments, transition through a multi-domain state is unavoidable during sensing. This could lead to higher noise level and reduce the sensitivity of the GMR sensor. This becomes more and more serious as the sensor size reduces and the sensor edge region occupies a larger and larger proportion of the total GMR sensor area.
One simple alternative is to replace the hard bias layer with an exchange biased magnetic layer as shown in FIG. 2. Seen there is soft magnetic layer 24 which is permanently magnetized through exchange coupling with antiferromagnetic (AFM) layer 25. Due to the abrupt cut of the junction edge in this scheme, it does not show a significant advantage relative to the hard bias scheme. To ensure magnetic continuity in the GMR sensor, it is preferred that the top magnetic layer not be touched during processing.
There are other schemes utilizing exchange bias to stabilize the GMR sensor edge. However, they all require extreme process control, like a few angstroms level etch control in order to be implemented. In the present invention we disclose a different approach, which provides a certain degree of specular reflection, a relatively large process window, a high exchange bias and convenience of integration into current existing process capabilities.
A routine search of the prior art was performed with the following publications of interest being found:    1. S. S. P. Parkin, “Systematic Variation of the Strength and Oscillation Period of Indirect Magnetic Exchange Coupling through the 3d, 4d, and 5d Transition Metals”, Phys. Rev. Lett., Vol. 67, P. 3598, 1991.    2. B. Dieny, V. S. Speriosu, S. S. P. Parkin, B. A. Gurney, D. R. Wilhoit and D. Mauri, “Giant Mangnetoresistance in soft ferromagnetic multilayers”, Phys. Rev. B, Vol. 43, P. 1297, 1991.
The following patents were also encountered during our search.:
In U.S. Pat. No. 6,266,218, Carey et al. show a GMR with a Bottom SV and patterned exchange process. U.S. Pat. No. 5,637,235 (Kim) discloses a BSV while U.S. Pat. No. 6,185,079 (Gill) shows an exchange biases DSV. U.S. Pat. No. 5,856,897 (Mauri) is a related GMR with AFM and FM layers. In U.S. Pat. No. 6,118,624, Fukuzawa et al. discuss abutted junctions and in U.S. Pat. No. 6,313,973 Fuke et al. describe laminated exchange coupling.